Friday, 29 April 2016

Nature is ruthless. It gives life and make stars sparkle so brightly in our sky that uncountable poets have dedicated their most beautiful work to them. But unfortunately, all beauty must fade and everything that has a beginning also has an end. Even so the seemingly perpetual stars which eventually have to die too. I've repeatedly written about dying stars, either the ones that go fairly quietly through the formation of a planetary nebula, or the ones that grant us the unforgettable spectacle of a supernova explosion. Today, I'd like to show you a star that's literally exhaling its dying breath.

I know, it's very difficult to see on this sketch and it's all my fault. I had made some modifications to my telescope which had the nasty consequence that I couldn't increase magnification beyond 206x anymore. Unfortunately, to have a good look at this little buddy you should use a lot more. But nevertheless you can still more or less see what I'm talking about: that rather bright, fuzzy little patch near the drawing's centre. This is a so-called protoplanetary nebula, nicknamed "Frosty Leo", and this nickname isn't far-fetched at all as I shall explain.

When a small to medium-sized star reaches the end of its lifecycle, it runs out of fuel to sustain nuclear fusion and becomes highly unstable. Its interior collapses and the shockwave that this causes literally blows the star's atmosphere into space, where it will form large gaseous shells or "bubbles" around the remains of the star. The contraction of the dying star's core will in turn generate so much heat that it will reignite fusion of helium into heavier elements, such as carbon, oxygen and even iron. The star's radiation continues to blow up the "bubble", which eventually dissipates into space, and heat it up to a point where the gas ions start to emit light as well. This is what we call a "planetary nebula".

In the case of Frosty Leo, however, we're not quite there yet. We're actually witnessing the collapse of the star and the initial expulsion of its atmosphere. Its last breath, as a matter of speaking. At this low magnification it's almost impossible to see, but the star's atmosphere is blown away in two opposite lobes which keep expanding at a rate of a whopping 25km/s. Remember that in order to escape from Earth's gravity a rocket needs an initial velocity of 11,2km/s or 33 times the speed of sound, so imagine how fast the nebula around Frosty Leo is forming!

As I said, the nickname wasn't chosen by chance or after a very successful party its discoverers had to celebrate their findings. No, the nickname derives from the fact that the nebula consists for a large part of... water-ice grains! Plus of course that it resides in Leo. For the time being it's perhaps the only such nebula that we know of so this makes it doubly interesting. Another weird fact is that it lies 10.000 lightyears away from us and an unusual 3.000 lightyears above the galactic plane. Therefore it must have been a very lonely star, condemned to die in complete isolation.

In the next millennia Frosty Leo will keep expanding and eventually the nebula, which currently only reflects the light from the star, will light up, adding another Crystal Ball or Eskimo to our skies. But let's not be impatient. This object is already a great spectacle and much more so from a scientific standpoint.

Friday, 15 April 2016

Betelgeuse, Orion's right shoulder, is a most fascinating star. Not only is it the 8th brightest star in the sky but also its strong, red colour has inspired all cultures around the world since the dawn of civilisation. As I've explained before, a red colour means that a star's actually quite cold. The temperature on its surface is barely 3.000°C, which is 2.500°C cooler than our Sun. But... Betelgeuse's also one of the largest stars that we know. It's so large that if it were at the position of our Sun, its surface would extend beyond Jupiter's orbit. Or to give you a better idea of its size... imagine that the Earth is a grain of sand with a diameter of 1mm. In comparison, our Sun would be the size of a grapefruit. And Betelgeuse would be... Wembley Stadium! That's how big it is! Then again, although Betelgeuse could contain 1,6 billion suns, its mass is estimated to be only 20 solar masses, making it an extremely low-density star. What's more, Betelgeuse is rapidly losing a lot of its mass. Observations with our most powerful telescopes revealed that it's ejecting enormous plumes of gas. Its surface is also terribly unstable and scientists believe that the star's contracted more than 15% over the last 20 years. Also its brightness shows big variations; the biggest of any bright star in our sky. In 1927 it only appeared to be the 20th brightest star but sometimes it outshone Rigel and Procyon to become the 6th brightest.

The reason for all of these spectacular figures is that Betelgeuse's arrived at the end of its very short life. Also this is hard to believe if you consider that our Sun's 4,6 billion years old and only halfway its lifecycle, but Betelgeuse's only 10 million years old! It was born long after the age of the dinosaurs, in the period that the big apes appeared and the flora and fauna on Earth started to take their present shape. The greater the mass of a star, the faster it consumes its energy source and eventually dies. We believe that Betelgeuse was born in the Orion Nebula complex as a star with a very large mass which burnt up very quickly. When its core ran out of hydrogen, it started to fuse helium into oxygen and carbon, greatly expanding the star's radius and causing the star to cool down. Claudius Ptolemy described Betelgeuse as ruddy in the 1st century AD, but interestingly, Chinese astronomers who observed the star 3 centuries earlier, said it was yellow. So we could conclude that the cooldown from a yellow to red supergiant star occured very recently.

Another interesting fact is that Betelgeuse's shooting through our universe at a speed of 30km/s, more than 3 times the speed a rocket needs to escape Earth's gravity or 90 times the speed of sound! As such, it's creating a shockwave that distorts space around it. Gas and dust are blown away and ripple in its wake and this is exactly what I wanted to show you on my sketch. I know, it's very difficult to see. Believe me, I had a hard time seeing it through my binoculars but eventually I did notice a kind of weird, dark triangle surrounding the star. This triangle's known as Betelgeuse's Ring and they are clouds of dust blown away by the quickly approaching giant star. So indeed, these clouds are dark because they're closer to us than the star and so they partly block its light.

But as I said, Betelgeuse's fast life of rock 'n roll will soon come to an end. It's already terribly unstable. Cooldown's reached a critical point. Iron's building up in its core. And then... it will explode in a gigantic supernova explosion, probably within the next 100.000 years already. It will be brighter than a full Moon and last for several months! But don't worry, there's no need for panic. Betelgeuse's between 500 and 600 lightyears away and although some lunatics have predicted doom or even judgement day, the radiation from this explosion will be far less than the radiation we receive from our Sun, so we'll be quite safe.

Wednesday, 6 April 2016

In one of my previous posts I talked about tree lovely galaxies that the lioness bares in her belly. But if we move slightly towards her chest (wow... soon I can start writing romance novels... :-) ) we find more beauties. On this sketch you can see the M96 group of galaxies. In reality this group is still much larger than what I could catch with the already enormous field of view of my binoculars and it contains at least 24 members, 4 of which are visible here.

M96 is the brightest of them all and proudly rests at the centre of my sketch. It's a spiral galaxy with rather weak but still clearly visible spiral arms, about the same size as our own galaxy. Measurements revealed that its gravitational centre is not in the middle but a bit off due to gravitational interaction with the other members in the group. Interesting to note is that ultraviolet emissions from the core suggest that it contains a supermassive black hole!

M95 can be found on the right. It's a galaxy with a fairly strange shape because its spiral arms appear to form a ring around the centre, rather than to originate from it. In reality it's a "bar-type" galaxy, which means that it has a bar-shaped structure across its centre from which originate the spiral arms, but its bar is rather weak and hardly visible. It certainly wasn't visible through my binos although I could see a hint of the ring of spiral arms. The nucleus itself is surrounded by an enormous star-forming region some 2.000 lightyears across.

Then we move to the little group on the top-left. The brightest member of this subgroup is denominated M105. It's an "elliptical" galaxy, meaning that it lacks the spiral arms and structure of spiral or lenticular galaxies. Once it was believed that these smooth and shapeless galaxies were young and that they'd evolve into spiral galaxies, but this turned out to be false. On the contrary, star formation is very low in them and most stars are much older than the stars found in spiral galaxies. However, it contains an even bigger black hole that the one in M96: it has a mass of 2*108 solar masses!

To the left of M105 we find its closest companion, denominated NGC3384. It's a lenticular or disk-shaped galaxy. These are somewhat intermediate between elliptical and spiral galaxies and together with the former they tend to be quite old with little star formation going on, although they do have a much clearer structure. Over 80% of the stars in this galaxy were found to be more than a billion years old. Invisible to amateur telescopes but still very interesting is that NGC3384 and M105 share a large ring of neutral hydrogen, 650.000 lightyears in size, in which star formation has been observed.

Last but not least, I present you a challenge. There's a small, third galaxy near the M105 subgroup and I hope that you can find it. It was extremely difficult to see through my binos anyway so I wanted to let you suffer just as much as I did when I was trying to locate it. :-) It's referred to as NGC3389 and although it appears to be a member of this subgroup and hence as a member of the M96 galaxy cluster, it isn't part of it at all! It lies twice as far away from us and is therefore a remote background object that only coincidentally seems to reside close to M105 and NGC3384. It's also a completely different type of galaxy than the other two, with a clear spiral structure and a high emission of blue light which indicates a huge quantity of active hydrogen and a lot of very young stars. But unfortunately this little galaxy ended up in a remote and isolated part of our universe, without any close companion.

The M96 cluster lies at a distance of 32 million lightyears and is in turn part of the Virgo supercluster of galaxies. This supercluster is truly gigantic as it not only contains the already vast and crowded Virgo cluster, but also the M101 group, the M82 group, the M65 group, the M51 group, the Draco group and... our own local group of galaxies. The supercluster's a whopping 110 million lightyears across and is defined as such because there is gravitational interaction between its various members, however distant. And here's the good news: there are millions of these superclusters in our universe. How about that for perspective?